On thermo- and photo-induced symmetry-broken transformation in spin-crossover complex: Cooperative activation

We study energetics of a cooperative low-spin (L) to high-spin (H) transformation in a crystal with two sublattices, where thermo- and photo-induced symmetry-broken phases have been recently observed. The model takes into account an intra-dimer antiferromatic coupling and inter-dimer couplings. Cooperative activation processes for switching between LH and HL states and that from LL to HH states of a dimer are analyzed. It is shown that locally (within a dimer) preferred symmetry-broken, LH and HL, states compete with inter-dimer couplings, which stimulates dynamical disorder via a decrease in the energy barrier for a flip between asymmetric configurations. On the other hand, a locally non-preferred symmetric state (HH) is supported by inter-dimer interaction via a cooperative activation mechanism. We conclude that symmetry-broken states can form an intermediate thermo-induced phase or can be observed as a photo-induced one at low temperatures as a result of local equilibrium. The global equilibrium follows from cooperative interaction and governs the thermo-induced re-entrant phase transition.